A mass spectral method for the identification of the urine metabolite of THC, 11-nor-delta-9-tetrahydrocannabinol-9-carboxylic acid, has been developed. The metabolite is extracted from the urine, derivatized to the methyl ester, methyl ether, and analyzed on a gas chromatograph/mass spectrometer. Identification of the metabolite is based on the retention time of the derivatized compound and its electron impact (EI) or chemical ionization (CI) mass spectrum. The EI spectrum shows three characteristic ions at masses 313, 357, and 372 (the molecular ion). When methane is used as the reactant gas, the CI spectrum shows an ion at 341 and an M + 1 ion at mass 373.